This invention relates to a flow modifier for aqueous systems which is also an effective adsorbate onto solids from aqueous systems. The modifier is a homopolymer or copolymer of acrylamide polymerized by a process employing an initiator or a chain transfer agent at least one of which comprises a hydrophobic group of an effective size. It is believed that the acrylamide polymer bears the hydrophobic group, probably as its terminal group or groups. Aqueous solutions of such a polyacrylamide have higher viscosities than aqueous solutions of polyacrylamide of similar molecular weight made by a process which does not entail hydrophobic groups, when measured at concentrations such as 0.1% or more. Multiphase compositions, comprising a continuous aqueous phase and at least one other phase, containing the polyacrylamide flow modifier have a surprising combination of desirable properties in a number of uses. Typical of these uses are paint and printing ink modification, emulsion stabilization, and pigment dispersion.
High molecular weight polyacrylamide and partially hydrolyzed derivatives thereof have long been known as thickeners for water and in various other uses as reported in "Handbook of Water Soluble Gums and Resins" R. L. Davidson, Ed., McGraw Hill 1980, Chapter 16 by Volk and Friedrich. Processes for preparing polyacrylamide are old in the art; Minsk et al., U.S. Pat. No. 2,486,191, teach solution polymerization in aqueous alcohol solution, Monagle et al., U.S. Pat. Nos. 3,336,269, 3,336,270 and 3,509,113, teach a precipitation process employing tertiary butanol, and optionally acetone, in water as the polymerization medium. Mercaptans are well known free radical polymerization modifiers and chain transfer agents and are so employed by Uraneck et al., U.S. Pat. No. 2,888,442, and O'Brien, U.S. Pat. No. 3,028,367.
Mercaptans as components of initiators for acrylamide polymerizations are taught by Chujo, U.S. Pat. No. 3,627,820, and Hicks, U.S. Pat. No. 3,960,824. Hicks teaches the preparation of the low molecular weight polymers, of monomers including acrylamide, by a bulk polymerization process using organic mercaptans, at about 0.01 to 0.3 equivalents per mole of monomer, in the presence of oxygen. The polymers are typically viscous liquids and tacky solids and may be used as plasticizers or components of 100% solids coatings and molding compositions. Chujo teaches a process for polymerizing monomers, including acrylamide, in the presence of a catalyst composition consisting of a complex compound of acetyl acetone with trivalent manganese and a mercaptan. Chujo's process and polymers produced thereby are discussed further below.
The course of a free radical polymerization is believed to entail the following principal reaction steps, where a ".degree." indicates the unshared electron of a free radical:
______________________________________ I .fwdarw. R.degree. Initiator I decomposes R.degree. + M .fwdarw. RM.sub.1 .degree. Initiation of Monomer M RM.sub.1 .degree. + M .fwdarw. RM.sub.2 .degree. Propagation RM.sub.n .degree. + M .fwdarw. RM.sub.(n+1) .degree. Step n + 1 RM.sub.n .degree. + RM.sub.m .degree. .fwdarw. RM.sub.(n+m) R (combination) Termination RM.sub.n .degree. + RM.sub.m .degree. .fwdarw. RM.sub.n H + RM.sub.m (-H) (disproportionation) " ______________________________________
Molecular weight of the polymer depends on the relative rates of initiation, propagation and termination. Termination is by: (1) combination, i.e., two radicals combining, (2) disproportionation, i.e. one radical abstracting a hydrogen atom from another to result in one saturated and one unsaturated molecule, often unreactive, or (3) transfer of the free radical to some other species present:
______________________________________ RM.sub.n .degree. + M .fwdarw. RM.sub.n + M.sub.1 .degree. With monomer RM.sub.n .degree. + RM.sub.m .fwdarw. RM.sub.n + RM.sub.m .degree. With dead polymer RM.sub.n .degree. + S .fwdarw. RM.sub.n + S.degree. With solvent or chain transfer agent S ______________________________________
The S.degree. then becomes an initiating radical to produce polymer molecules such as SM.sub.n, SM.sub.(n+m) S and SM.sub.(n+m) R.
Some of the products of transfer may be unreactive radicals. Use of chain transfer to limit molecular weight is old, commonly employing mercaptans, secondary alcohols or other molecules with an easily abstracted H atom. An initiator fragment and a chain transfer fragment are thus the end groups of many or all of the polymer molecules.
It is believed that the polyacrylamide of the instant invention comprises one or more terminal alkylmercapto groups wherein the alkyl group is of a size large enough to tend to drive the polyacrylamide out of water solution, i.e. be effective hydrophobes. This tendency is believed to be energetically favored by the formation of water-water secondary bonds made possible if the alkyl groups associate. This tendency may be responsible for many of the unusual and unexpected properties of the polyacrylamides of the instant invention and their aqueous solutions. Among these properties is the surprising increase in viscosity of an aqueous solution as the concentration of the polymer is increased as compared to a polymer employing a less water insoluble mercaptan chain transfer agent or polymerization modifier. Similarly, unusual behavior is found in the rheological properties of emulsions, suspensions and other multiphase systems, such as those containing both liquid and solid phases (mixed emulsions and suspensions), comprising this polyacrylamide. Further novel effects are noted when systems containing the polyacrylamide of the instant invention are dried and ease of bonding to the exposed surface is determined. It is seen that bonding is poor, especially the bonding between two coated surfaces (i.e., so-called "blocking") and it is concluded that these polyacrylamides are surprisingly adhesion-inhibiting adsorbates. The thickeners or flow modifiers of the invention are exceptionally versatile in their ability to impart special rheological properties to aqueous solutions and to multiphase solutions comprising an aqueous continuous phase. They are relatively efficient viscosity improvers at comparatively low molecular weights and at the same time provide improved rheological properties to aqueous solutions and systems subjected to high or low shear rate. An example of these improvements is found in latex paints containing the modifiers of this invention formulated so as to provide, in a single paint formulation, superior flow and leveling, and desired viscosity values under both low shear and high shear conditions as well as result in coatings having high gloss, and resistance to blocking.
Water soluble thickeners are notoriously difficult to handle in industrial processes because when furnished as solids they take inordinately long times to dissolve, or when furnished as concentrated solutions they are difficult to dilute uniformly so as to avoid the production of local areas of high concentration, often known as "fish-eyes" because of their appearance. Both in simple aqueous solution and, more particularly, in formulations comprising other phases, extended agitation and aging periods are required before an equilibrium viscosity is attained. This requires formulators to guess or develop rule of thumb methods for estimating the final equilibrium rheological properties of the formulation or to inventory large amounts of product and reprocess it if the final rheological properties do not meet an established specification. One of the advantages of the polyacrylamides of this invention is the rapidity with which equilibrium rheological properties are attained. This rapid equilibration may be due to the relatively high viscosity of the aqueous solutions, with respect to the molecular weight of the polyacrylamide, exhibited by the water solutions having concentrations in excess of 0.1%. Thus one may regard the aqueous solutions as having a viscosity to be expected of high molecular weight polyacrylamides while, at the same time, insofar as diffusional characteristics, such as concentration equilibration rate, are concerned, the polyacrylamide behaves as though the molecular weight is comparatively low. This is both a surprising property and one of great importance in the manufacture of multiphase systems.
The rapid equilibration, in the distribution in solution, of the polyacrylamides of this invention may be considered to be associated with the unusual rheological properties of systems thickened thereby. Without being bound by the theories expressed or implied, the following remarks are offered as an aid in the teaching of this invention. The introduction of thickener polymers into a solution is thought to promote a specific structure in the solution phase or an interaction among phases resulting in a structure. If the structure is resistant to change, throughout a broad range of shear rates, then its rheology profile is Newtonian, that is, its apparent viscosity is relatively independent of shear rate. If the structure is not shear resistant, but degrades under shear and does not reform as fast as the degradation, then its rheological profile will be less Newtonian and more pseudoplastic. A water soluble high molecular weight polymer added to an aqueous phase readily gives an increased viscosity as measured to a given low shear rate. However, as the shear rate is increased, it is commonly observed that the viscosity diminishes, that is, the solution appears to have pseudoplastic rheology. This may be due to the original structure not being maintained because the high molecular weight polymer molecules do not permit rapid equilibration of their distribution in the solution. Because of the unexpectedly high rate of equilibration, the thickeners of this invention exhibit a more nearly Newtonian rheology then do current commercial thickeners, even at high shear rates.
The nonionic polyacrylamides of this invention have outstanding tolerance for electrolytes in the sense that the rheology properties of their aqueous solutions are comparatively insensitive to electrolytes as well as to changes in pH. The viscosity of aqueous solutions thickened by these polyacrylamides is less sensitive to shear than solutions of the same viscosity thickened by prior art materials.
The acrylamide polymers prepared with hydrophobe containing chain transfer agents and optionally hydrophobe containing initiators function as thickeners and rheology modifiers in aqueous systems at concentrations greater than about 0.1% in water. Formulated with a wide variety of aqueous systems comprising small particles, such as latexes, these polymers afford multiphase systems of great commercial interest such as paints, printing inks and other systems comprising a thickened aqueous phase. An example of such materials are gloss paints which are found to have an excellent balance of low shear and high shear viscosity, exceptional flow, leveling and film build and which produce resulting paint films having high gloss, block resistance, and, when compared to formulations comprising conventional nonionic thickeners such as hydroxyethyl cellulose or anionic thickeners, have better alkali and abrasion resistance and resistance to degradation such as by shearing or by enzymes. In general, the polyacrylamides of the instant invention are also effective sedimentation stabilizers, surfactants, and dispersants.